<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin:0;
            padding:0;
        }
        body{
            display: flex;
        }
        .leftbox{
            width:200px;
            height:100vh;
            background-color: aquamarine;
        }
        .rightbox{
            flex:1;
            height:100vh;
            background-color: pink;
        }
    </style>
</head>
<body>
    <div class="leftbox"></div>
    <div class="rightbox"></div>
</body>
</html>